i965: Avoid unnecessary recompiles for shaders that don't use dFdy().

The i965 back-end needs to compile dFdy() differently for FBOs and
window system framebuffers, because Y coordinates are flipped between
the two (see commit 82d2596: i965: Compute dFdy() correctly for FBOs).
This patch avoids unnecessarily recompiling shaders that don't use
dFdy(), by only setting render_to_fbo in the wm program key if the
shader actually uses dFdy().

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -2119,19 +2119,13 @@ brw_fs_precompile(struct gl_context *ctx, struct gl_shader_program *prog)
    struct brw_context *brw = brw_context(ctx);
    struct brw_wm_prog_key key;
 
-   /* As a temporary measure we assume that all programs use dFdy() (and hence
-    * need to be compiled differently depending on whether we're rendering to
-    * an FBO).  FIXME: set this bool correctly based on the contents of the
-    * program.
-    */
-   bool program_uses_dfdy = true;
-
    if (!prog->_LinkedShaders[MESA_SHADER_FRAGMENT])
       return true;
 
    struct gl_fragment_program *fp = (struct gl_fragment_program *)
       prog->_LinkedShaders[MESA_SHADER_FRAGMENT]->Program;
    struct brw_fragment_program *bfp = brw_fragment_program(fp);
+   bool program_uses_dfdy = fp->UsesDFdy;
 
    memset(&key, 0, sizeof(key));
 

src/mesa/drivers/dri/i965/brw_fs_emit.cpp

@@ -931,6 +931,10 @@ fs_visitor::generate_code()
 	 generate_ddx(inst, dst, src[0]);
 	 break;
       case FS_OPCODE_DDY:
+         /* Make sure fp->UsesDFdy flag got set (otherwise there's no
+          * guarantee that c->key.render_to_fbo is set).
+          */
+         assert(fp->UsesDFdy);
 	 generate_ddy(inst, dst, src[0], c->key.render_to_fbo);
 	 break;
 

src/mesa/drivers/dri/i965/brw_wm.c

@@ -441,13 +441,7 @@ static void brw_wm_populate_key( struct brw_context *brw,
    const struct gl_program *prog = (struct gl_program *) brw->fragment_program;
    GLuint lookup = 0;
    GLuint line_aa;
-
-   /* As a temporary measure we assume that all programs use dFdy() (and hence
-    * need to be compiled differently depending on whether we're rendering to
-    * an FBO).  FIXME: set this bool correctly based on the contents of the
-    * program.
-    */
-   bool program_uses_dfdy = true;
+   bool program_uses_dfdy = fp->program.UsesDFdy;
 
    memset(key, 0, sizeof(*key));
 

src/mesa/drivers/dri/i965/brw_wm_emit.c

@@ -1757,6 +1757,10 @@ void brw_wm_emit( struct brw_wm_compile *c )
 	 break;
 
       case OPCODE_DDY:
+         /* Make sure fp->program.UsesDFdy flag got set (otherwise there's no
+          * guarantee that c->key.render_to_fbo is set).
+          */
+         assert(c->fp->program.UsesDFdy);
 	 emit_ddxy(p, dst, dst_flags, false, args[0], c->key.render_to_fbo);
 	 break;